Olefin polymerization process

ABSTRACT

An olefin polymerization process for the preparation of polyolefins having relatively low molecular weights and high melt indexes without loss of catalyst activity, utilizing a catalyst comprising a silica xerogel support having a pore volume greater than about 2.0 cc/g, the major portion of which volume is provided by pores having pore diameters ranging from 300 to 600 A and a surface area ranging from 200 to 500 m 2  /g, and having deposited thereon a metal-containing catalytic material, such as chromium oxide, or other metal oxide. The process is designed for the preparation of polymers and copolymers of 1-olefins having a maximum of 8 carbon atoms in the chain and having no branching nearer the double bond than the 4-position, e.g., ethylene polymers.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of copending application Ser.No. 383,203, (now abandoned) filed July 27, 1973 which, in turn, is acontinuation of abandoned application Ser. No. 122,503 filed Mar. 9,1971 as a division of abandoned application Ser. No. 750,467 filed Aug.6, 1968.

Further, this application is a continuation-in-part application Ser. No.437,274, filed Jan. 28, 1974, which is a continuation-in-part ofabandoned application Ser. No. 294,270, filed Oct. 10, 1972 which, inturn, is a continuation-in-part of abandoned application Ser. No. 70,622filed Aug. 14, 1970 and which, in turn, was a continuation-in-part ofeach of applications Ser. Nos. 750,733 filed Aug. 6, 1968 (now U.S. Pat.No. 3,652,214), Ser. No. 750,734 filed Aug. 6, 1968 (now U.S. Pat. No.3,652,215), and Ser. No. 766,693 filed Oct. 11, 1968 (now U.S. Pat. No.3,652,216).

This application is also a continuation-in-part of copending applicationSer. No. 486,788, filed July 9, 1974, which is a continuation-in-part ofabandoned application Ser. No. 326,645, filed Jan. 26, 1973 which, inturn, is a continuation of abandoned application Ser. No. 148,117 filedMay 28, 1971 and which, in turn, was a continuation-in-part of theaforesaid application Ser. No. 750,734 filed Aug. 6, 1968. Copendingapplication Ser. No. 486,788 is also a continuation-in-part of abandonedapplication Ser. No. 311,579, filed Dec. 4, 1972 which, in turn, was acontinuation of application Ser. No. 122,502, filed Mar. 9, 1971 andSer. No. 750,467, filed Aug. 6, 1968.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the polymerization of 1-olefinshaving a maximum of 8 carbon atoms in the chain and having no branchingnearer the double bonds than the 4-position. The polymerization processis carried out in the presence of a catalyst formed on a silica xerogelsupport having a cumulative pore volume, pore diameter distribution, andsurface area such that relatively low molecular weight, high melt indexpolyolefins are readily produced in the particle form polymerizationreaction.

2. The Prior Art

In recent years new processes and catalysts for the stereospecificpolymerization of olefins have received considerable attention.Catalysts having stereospecific activity include metal-containingcatalytic materials, e.g., chromium oxides, deposited on silica, oralternatively alumina, supports which have previously been activated byoxidation at elevated temperatures. Olefins may be polymerized with suchcatalysts to produce a varied series of polymers having differingmolecular weights and melt indexes, depending upon the particulartemperatures, pressures, solvents or other diluents, catalysts, or otherreaction conditions used.

For many applications the production of low molecular weight, high meltindex polymers is of particular advantage, such materials findingimportant applications in films and sheets, extrusion coating, injectionand rotational molding, and the like. Considering the preparation ofethylene polymers as illustrative, low molecular weight (high meltindex) polyethylenes are commercially obtained by carrying out thepolymerization in solution (the "solution process"), but only withconversions of less than about 1000 pounds of polyethylene per pound ofsupported catalyst (equivalent to ≧10 ppm as Cr on a polymer basis as Crcontent of supported catalyst is ≧1%). On the other hand, when thereaction is carried out in suspension (the "suspension" or "particleform" process), it is possible to obtain conversions of from about 5000to 15000 pounds of polyethylene per pound of supported catalyst havingCr content of ≦1% (equivalent to ≦2 ppm Cr on a polymer basis).Moreover, it is necessary in the solution process, in order to preservethe color and desired appearance of the product resin, to maintain thechromium content in the resin lower than about 2.5 ppm. The catalystmust, therefore, be removed from the polymer product formed in thesolution process. The catalyst need not, however, be so removed duringparticle form processing. The particle form process thus exhibitsdistinct commercial advantages relative to the solution process for thestereospecific polymerization of olefins.

Heretofore, however, particle form or slurry operations have beenlimited, at high conversion rates equal to or greater than about 5000pounds of polyethylene per pound of catalyst, to the production ofpolyolefins having melt indexes lower than about 2. Various techniqueshave been proposed to increase the melt indexes of olefin polymers soproduced, with varying degrees of success. For example, while the use ofmodifier such as hydrogen has been found to decrease the molecularweight and increase the melt index of the polymer product, theadvantages attendant the use of such materials are limited since theysimultaneously decrease catalyst activity. Similarly, variation of thechromium oxide content of the catalyst, addition of different metaloxide promoters, combination of different supports and/or the use ofvarying catalyst activation temperatures, have been widely investigated,with only marginal improvement.

Modification of the porosity, surface area and other characteristics ofthe catalyst support has also been suggested as a means for increasingthe melt index of olefin polymers produced by particle formstereospecific polymerization reactions. Thus, in recent years a numberof procedures have been described in the literature for the preparationof silica gel materials said to be useful as catalyst supports for thispurpose. Such procedures are described, for example, in U.S. Pat. Nos.3,132,125 and 3,225,023; and in British Patent Specification No.1,007,722. Silica gels so prepared have not, however, achieved theirintended purpose, i.e., the production of olefin polymers havingmarkedly increased melt indexes.

Thus, for example, Schwander et al U.S. Pat. No. 3,132,125 describes theuse in both solution and suspension processes of stereospecificcatalysts supported on non-porous silicas for the production ofpolyolefins said to have relatively low average molecular weights and,correspondingly, high melt indexes. Relatively high melt index polymerswere in fact produced in the solution phase operations exemplified bySchwander et al. Where, however, particle form operations were utilizeduse of the catalyst described in this patent resulted in the preparationof polymer products having melt indexes (estimated from the molecularweight data set forth by Schwander et al) no greater than about 0.2.

Hogan et al U.S. Pat. No. 3,225,023, assigned to Phillips PetroleumCompany, suggests that olefin polymers having increased melt indexes maybe produced employing catalyst supports having increased average porediameters, ranging from about 60 to 400 A. Hogan et al illustrate theirprocess by experimental runs (which may have been conducted in eitherthe solution or suspension phases), employing "commercial silica gel"supports having varying average pore diameters. The use of silica gelsof the type commercially available as of the Hogan et al filing date(November, 1962) and having the indicated range of average porediameters has not, however, resulted in the formation of very high meltindex polymers employing particle form operations. Thus, polyethylenesso produced (employing chromium oxide catalysts deposited on suchsupports) have melt indexes of only up to about 3.0.

British Patent Specification No. 1,007,722, also assigned to PhillipsPetroleum Company, describes the use of "a specific form of high purityfinely divided porous silica gel" as a support for a chromium oxidecatalyst said to be capable ofproducing relatively high melt indexpolyethylenes in a particle form polymerization. The specific form ofsilica gel referred to in the British specification is a silica aerogelhaving a pore diameter between approximately 200 A and 500 A, a surfacearea of approximately 250 to 350 m² /g, a density of less thanapproximately 0.2 g/ml., and an oil adsorption of approximately 300/100lbs. "Syloid" 244 (having a surface area of 250 m² /g, a pore volume of2.2 cc/g, and a pore diameter of 350 A) is the sole such materialexemplified.

Aerogels are silica gels in which the liquid phase has been replaced bya gaseous phase in such a way as to avoid shrinkage as occurs by directevaporation of the liquid phase thereof (materials prepared in thelatter manner being termed xerogels); Iler, The Colloid Chemistry ofSilica and Silicates, University Press, pages 137 and 152. Aerogels are,however, subject to subsequent shrinkage when wetted due to coalescenceof their ultimate particles. Shrinkage of this nature decreases porosityand markedly impairs the use of these materials as stereospecificcatalyst supports. Moreover, aerogels readily disintegrate whensubjected to mechanical stress. Thus, it has been found that the use ofsilica aerogels as catalyst supports in the particle form process isless than satisfactory.

Nor have other recently disclosed silica gel materials having varyingporosity and surface area characteristics proved adequate to effect theproduction of high melt index olefin polymers in particle formoperations. Such materials are disclosed, for example, in U.S. Pat. Nos.2,731,326; 3,403,109; 3,428,425; and 3,669,624; and in British PatentSpecification No. 1,077,908.

As illustrative, Hyde U.S. Pat. No. 3,453,077, and British PatentSpecification No. 1,077,908, both of which are assigned to W. R. Graceand Co., disclose methods said to result in the peparation of"microspheroidal silica gels" having pore volumes within the range offrom as low as 0.3 cc/g (the British specification) to as much as 2.5cc/g (the U.S. patent), and surface areas within the range of from 100to 800 m² /g. These references describe procedures for the preparationof silica gels involving gelling alkali metal silicate solutions withgaseous carbon dioxide or mineral acids, neutralizing either about half(the British specification) or substantially the entire alkali metalsilicate content of the hydrogels thus formed, aging the neutralizedgels (and, in the case of the U.S. patent, making the gel pH alkalinewith ammonium hydroxide), thereafter spray-drying the hydrogel to removethe liquid phase, washing the spray-dried material and re-drying thesame for subsequent use. It has, however, been found that theseprocedures do not enable one to prepare silica gel materials havingcumulative pore volumes as large as 2.0 cc/g. Moreover, when silica gelsthus made are used as supports for stereospecific catalysts in theparticle form polymerization of ethylene, polyethylenes having meltindexes of only up to about 2 are obtained.

From the preceding it will be seen that prior efforts to producerelatively high melt index olefin polymers in particle form operationsby the use of modified silica gel catalyst supports and/or othertechniques have not been entirely satisfactory. It is, therefore, aprincipal object of the present invention to provide an particle formolefin polymerization process utilizing an improved silica gel-supportedstereospecific catalyst, which effects the production of polymers havingsubstantially higher melt indexes than heretofore obtained in suchoperations. Other objects and advantages of the present invention willbe apparent from the following description of the nature and preferredembodiments of the polymerization process hereof.

SUMMARY OF THE INVENTION

In accordance with the present invention an improved particle formpolymerization process is provided, utilizing a catalyst comprising asilica xerogel support having a metal-containing catalytic materialdeposited thereon. The silica xerogel has a pore volume greater than1.96 cc/g, i.e., greater than about 2.0 cc/g, the major portion of whichpore volume is provided by pores having average pore diameters withinthe range of from about 300 to 600 A; and a surface area within therange of from about 200 to 500 m² /g. The pore volume of the xerogel issuitably provided by pores having a narrow pore diameter distributionprimarily within the indicated 300 to 600 A range. The metal-containingcatalytic material deposited on the support is preferably a metal oxide,especially chromium oxide or another metal oxide such as cobalt, nickel,vanadium, molybdenum or tungsten oxides. It has been found that astereospecific catalyst comprising the specified silica xerogel supporthaving the indicated cumulative pore volume, average pore diameter andsurface area characteristics is quite effective in particle form olefinpolymerization in producing olefin polymers having markedly hgiher meltindexes than heretofore obtained.

The polymerization process of the invention comprises contacting a1-olefin having a maximum of 8 carbon atoms in the chain and nobranching nearer the double bond than the 4-position (preferablyethylene) with the foregoing catalyst under polymerization conditions toprovide the indicated hgih melt index, low molecular weight polyolefinproducts. For example, employing such catalyst in the particle formpolymerization process hereof polyethylenes may be readily produced withmelt indexes in excess of 2.0, and up to about 15.

Preferably, the silica xerogels employed in the catalyst hereof havecumulative pore volumes ranging from about 2.0 to 3.5 cc/g, with about70% or more of the pore volume being provided by pores having an averagepore diameter within the approximate 300 to 600 A range. Use of suchmaterials as supports for chromium oxide-containing catalysts, forexample, results in the formation of polyethylenes having particularlyadvantageous, high melt indexes ranging from about 3 to 12.5.

The silica xerogel supports of the catalyst utilized in the practice ofthe present invention, and particularly the porosity and surface areacharacteristics thereof, are described in terms of their pore volumes(PV), surface areas (SA), and average pore diameters (PD). The surfacearea is determined by the standard BET method described by Brunauer,Emmett and Teller, J. Am. Chem. Soc., 60,309 (1938). The pore volume isdetermined by the well known nitrogen adsorption-desorption techniquedescribed, for example, in Catalysis, Vol. II, pages 111-116, Emmett, P.H., Reinhold Publishing Corp., New York, New York, 1955 (Run to a P/Poof 0.967 which is equivalent to 600 A pore diameter) and elsewhere. Thepore volumes referred to herein refer to the gel volumes determined bypermitting nitrogen gas to be adsorbed by and condensed in the pores ofthe gel at the normal boiling point of liquid nitrogen and at somerelative pressure P/Po, wherein P is the pressure of the nitrogen vaporover the gel and Po is the vapor pressure of liquid nitrogen. For silicagels, the determination of this nitrogen pore volume at a relativepressure P/Po = 0.967 permits computation of the volume of those poreshaving diameters of up to 600 A, which principally contribute to the gelsurface phenomena. The average pore diameter may be calculated from thisdata as follows: ##EQU1##

It should further be understood, that, as used herein, "pore volume" and"cumulative pore volume" are synonymous, and refer to the total volumeof the pores which comprise the xerogel structure per unit weightthereof. Similarly, the terms average or mean "pore diameter" or "poresize" are used interchangeably herein, and refer to a one-pointrepresentation of an actual distribution calculated by the above formulawhich is based on the geometric model of a right circular cylinder.

The pore volume of the xerogel catalyst support should be contrastedwith the water pore volume measurement occasionally used for theevaluation of silica gels and determined in accordance with the methodof Innes, Analytical Chemistry, 28, 332-4 (1956). The latter methodgives a result which sums the pore volume in all pores from the verysmallest through the macro-pores, or 0 to >1500 A diameter. This is incontrast with the N₂ adsorption method which when run to a P/Po of 0.967sums the pore volume in pores from 0-600 A diameter (i.e., micro-pores).Such a method (i.e., the water pore volume) is, therefore, notdiscriminating enough for measuring the pore volume of the silicaxerogels hereof, the major portion of which is provided by pores havingpore diameters within the range of from about 300 to 600 A. For suchmaterials it is rather necessary to determine pore volume by measurementof the volume of liquid nitrogen adsorbed per gram of the dry gel inaccordance with the technique known in the art (see also Barrett,Joiner, and Halenda, J. Am. Chem. Soc., 73, 373 [January, 1951]).

The silica xerogels employed in the supported catalyst are prepare inaccordance with the methods disclosed in the aforesaid U.S. Pat. Nos.3,652,214; 3,652,215; 3,652,216; 3,794,712; 3,794,713; and 3,801,705,respectively. The methods descried in the noted prior disclosures (whichare incorporated herein by this reference) involve the following stepsfor preparation of the silica xerogel:

1. Precipitating a silica hydrogel, under conditions of good agitation,by neutralizing an aqueous alkaline silicate solution, e.g., with astrong acid, a weak acids such as CO₂, an ion exchange resin, or byother suitable means to produce a silica hydrogel slurry, employing thefollowing conditions:

a. the neutralizing medium is added to the aqueous alkaline silicatesolution at a rate such that the gel point of the solution is reached infrom about 30-120 minutes, e.g., at a rate of up to 40% of the neededamount in 30-120 minutes and the remaining 60% in from about 20-90minutes more,

b. the temperature during precipitation is maintained between about 0°and 17°C,

c. the SiO₂ concentration in the final slurry is between about 5 and 12percent by weight, and

d. the final pH of the hydrogel slurry is from pH 3-8;

2. Maintaining the hydrogel slurry at a pH within the range of ph 3-8 ata temperature and for a time sufficient to strengthen the hydrogelsturcture;

3. Reducing the concentration of the alkaline material in the hydrogelby washing the same with a liquid which displaces the alkaline material,until the wash liquor recovered contains less than about 20 ppm of thealkaline matrial, expressed as salt thereof; and

4. Drying the resulting product, either by vacuum freeze-drying(specifically as described in the aforesaid U.S. Pat. Nos. 3,652,214 and3,794,712), solvent displacement (specifically as described in theaforesaid U.S. Pat. Nos. 3,652,215 and 3,794,713), or azeotropicdistillation (specifically as described in the aforesaid U.S. Pat. Nos.3,652,216 and 3,801,705).

The metal-containing catalyst material can be deposited on the silicaxerogel support thus formed in any suitable manner. For example, thesilica xerogel base can be coated with a metal oxide by shear mixing thefinely ground metal oxide with the silica base at room temperature. Whenchromium oxide is selected as the metal-containing catalyst, 0.5% to 5%by weight and preferably from 1% to 3% by weight based on the totalweight of the supported catalyst, may be deposited on the xerogelsupport. Similar results can be obtained by blending the metal oxide andthe carrier under vacuum conditions and/or under a nitrogen atmosphereat 200°C. Activation of the catalyst is carried out in air in afluidized bed at temperatures between about 1500° and 2000° F, andpreferably at about 1825° F. The period set for activation is on theorder of from 2 to 10 hours and preferably about 6 hours at theforegoing temperatures conditions. The activation is accomplishedwithout any physical change in the carrier.

The supported catalyst, e.g., a chromium oxide supported on the silicaxerogel support having the indicated characteristics, is employed inaccordance herewith in the particle form polymerization of 1-olefinshaving a maximum of 8 carbon atoms in the chain and no branching nearerto the double bond than the 4-position. The polymerization processhereof may be used for the preparation of homopolymers or copolymers oftwo or more 1-olefins of the foregoing type. In either case, theparticle form reaction is effected in a manner known per se asdescribed, for example, in Hogan et al U.S. Pat. No. 2,825,721.

PREFERRED EMBODIMENTS OF THE INVENTION

As indicated hereinabove, the particle form polymerization of thepresent invention results in the formation of unique polymer products;in the case of the polymerization of ethylene with chromium oxide-silicaxerogel catalysts, polyethylenes are produced having low molecularweights, evidenced by melt indexes of between about 2 to 15, and lowchromium levels, less than about 2.5 ppm chromium (without removingcatalyst residues from the system). Preferred embodiments ofpolymerization techniques thus useful may be found in the followingexamples, which are intended as illustraive only:

EXAMPLE I AND CONTROL A Preparation of Polyethylenes with Process of theInvention as Compared with Use of Prior Art Processes

This example is directed to the preparation of ethylene homopolymers asdescribed in the aforesaid application Ser. No. 750,467, now abandoned.The supported catalysts of Examples I-1, I-2, and I-3 were prepared andutilized in the particle form polymerization of ethylene in the mannerdescribed in Examples 2 (Examples I-1 and I-2 hereof) and 3 (ExampleI-3) of the aforesaid application. The control supported catalysts A-1through A-4 were commercially available "M.S. Catalysts" comprisingpre-formed chromium oxide (2.1%) on silica xerogels, activated andemployed in the particle form polymerization reaction as described inExamples 1 (Controls A-1 through A-3) and 3 (Control A-4) of theaforesaid Ser. No. 750,467.

In Examples I-1, I-2 and I-3 the respective silica xerogel catalystsupports were prepared as follows:

10,080 g of sodium silicate solution containing 28.7% SiO₂ and 8.9% ofNa₂ O was added to 12,720 g of water and cooled to 5° C., underagitation.

11,200 g of H₂ SO₄ (12.75 wt. %) was then added as follows:

a. 4480 g was added at a constant rate over a period of 1 hour, and

b. the remainder was added over a period of 45 minutes. The final pH ofthe precipitate was 6.2 and the SiO₂ content was about 8.5%.

The slurry was then heated to 95° C and held at that temperature for 3hours. The gel was washed with a solution of 1113 g of NH₄ NO₃ in 45gallons of water, and then with de-ionized water until the filtratetitrated less than 20 ppm Na₂ SO₄.

The product was reslurried in acetone and washed with acetone until thewater in the acetone titrated less than 1%.

The product was then homogenized and the acetone distilled off to reducethe acetone content to less than 1% by weight.

The silica gel obtained was calcined in an oven at 1000° F for 4 hoursbefore evaluation. The physical properties of the silica xerogel thusobtained were: pore volume (PV) = 2.66 cm³ /g surface area (SA) = 307 m²/g and an average pore diameter (Av.PD.) = 347 A. The xerogel was coatedwith 2.1% CrO₃, to have a chromium level comparable to the commerciallyavailable "M.S. Catalysts".

The coating was done by adding 813 g. of dry xerogel support and 16.45 gof dry powdered chromium oxide into a ribbon blender. A vacuum of 28inches of mercury was drawn on the blender and heat was applied so as toobtain a temperature of 250° C. in 3 hours. The heat was maintained for2 hours, and the catalyst was then brought to room temperature andstored in air-tight containers.

Portions of the catalysts thus prepared were calcined in a fluidized bedusing air flow rates of 0.2 feet³ per minute in an activator having a4-inch diameter. The catalysts were thus activated at temperatures of1750° F (Example I-1), 1825° F (Example I-2), and 1800° F (Example I-3).The respective maximum temperatures were maintained for 6 hours, afterwhich the individual catalysts were stored under nitrogen until used.

The thus activated catalysts were utilized in the particle formpolymerization ethylene within an 88 gallon loop reactor. Ethylenemonomer, isobutane solvent and the respective catalysts were fedcontinuously into the reactor to maintain ethylene saturation at 5% ± 1%and solids between 15 and 25 percent. The reactor temperature wasmaintained at about 230°-233° F, the reactor pressure at 650 psig, andthe polyethylene-containing slurry formed within the reactor wascirculated therethrough at a rate of from 15-25 feet per second. Thepolyethylenes thus produced were recovered and their melt indexes [byASTM D-1238-65T (Condition E)] , annealed densities and ash contentswere determined.

The properties of the silica gel catalyst supports and the activatedcatalysts, and the polymer properties obtained in the particle formpolymerization are set forth in Table I below for each of Examples I-1,I-2 and I-3.

Control experiments A-1 through A-4 were carried out to compare theproperties of polyethylenes produced employing the conventional "M.S.Catalysts". 250 g samples of the M.S. Catalysts were calcined in afluidized bed at activation temperatures of 1600° F (Control A-1), 1700°F (Control A-2), 1800° F (Control A-3), and 1550° F (Control A-4). Thethus activated control catalysts were stored under nitrogen until readyfor use, their pore volumes, pore diameters and surface areas weredetermined, and they were employed in the particle form polymerizationin the same manner as aforesaid. The properties of the activated M.S.Catalysts and the polymer properties obtained therewith are additionallyset forth in Table I.

The polymerization of Example I-3 and Control A-4 were carried out inthe presence of a hydrogen modifier (hydrogen concentration, mole ratioof H₂ /Et = 1 × 10⁻ ²). As will be noted from Table I, higher polymermelt indexes were obtained in the presence of the modifier. It will,however, further be noted that both in the polymerizations conductedwith and without the modifier, substantially higher polymer melt indexeswere achieved utilizing the present process, i.e., employing supportedcatalysts having the pore volume, pore diameter and surface areacharacteristics defined hereinabove, as compared with the M.S.Catalysts.

                                      TABLE I                                     __________________________________________________________________________    COMPARATIVE PROPERTIES OF POLYETHYLENES PRODUCED EMPLOYING THE PROCESS OF     THE                                                                           INVENTION AS COMPARED WITH PROCESSES EMPLOYING CONTROL CATALYSTS              Silica Gel Properties                                                                             Activated Catalyst Properties                                                                      Polymer Properties                   __________________________________________________________________________                        Pore Average Pore                                              PV   Av.PD                                                                              SA   Volume                                                                             Diameter                                                                              Surface Area                                                                          Milled                                                                             Annealed                                                                           Ash                        Example                                                                            (cm.sup.3 /g)                                                                      (A)  (M.sup.2 /g)                                                                       (cm.sup.3 /g)                                                                      (A)     (cm.sup.3 /g)                                                                         MI   Density                                                                            ppm                        __________________________________________________________________________    I-1  2.66 347  307  2.14 340     252     3.2  0.9645                                                                             164                        I-2  2.66 347  307  2.23 356     251     4.2  0.9647                                                                             184                        Control                                                                       A-1  --   --   --   N.A. N.A.    N.A.    1.3  --   211                        A-2  --   --   --   1.44 225     226     1.8  --   184                        A-3  --   --   --   1.25 259     193     1.6  --   330                        Polymerization Carried Out with Hydrogen Modifier                             Example                                                                       I-3  2.66 347  307  2.20 347     253     12.2 0.9668                                                                             305                        Control                                                                       A-4  --   --   --   1.55 260     234     3.2  0.9699                                                                             238                        __________________________________________________________________________

EXAMPLES II AND III AND CONTROLS B AND C Preparation of EthyleneCopolymers with Process of the Invention as Compared with Use of PriorArt Processes

Ethylene copolymers were produced in the presence of chromiumoxide-supported catalysts prepared as described in Example I and ControlA above, the control experiments employing the aforesaid M.S. Catalysts.In Example II and Control B the respective catalysts were activated at1800° F (Example II) and 1700° F (Control B), and were then utilized inthe copolymerization of ethylene with hexene-1 (0.5 wt. %) at about 220°F (Examples II -- 221.5° F; Control B -- 223° F). The properties of therespective xerogels and catalysts employed, and the ethylene-hexenecopolymers obtained are set forth in Table II below.

In Examples III and Control C the respective catalysts were activated at1800° F (Example III) and 1550° F (Control C) and were then utilized inthe copolymerization of ethylene with about 1.5 wt. % butene-1 (ExampleIII -- 1.6%; Control C -- 1.7%) at about 215° F (Example III -- 215° F;Control C -- 213° F). The properties of the respective xerogels and thecatalysts embodying the same, and the ethylene-butene copolymersobtained therewith, are also set forth in Table II.

                                      TABLE II                                    __________________________________________________________________________    COMPARATIVE PROPERTIES OF ETHYLENE COPOLYMERS PRODUCED EMPLOYING THE          PROCESS                                                                       OF THE INVENTION AS COMPARED WITH PROCESSES EMPLOYING CONTROL CATALYSTS       Silica Gel Properties                                                                            Activated Catalyst Properties                                                                     Polymer Properties                                        Pore Average Pore                                          PV        Av. PD                                                                             SA  Volume                                                                             Diameter                                                                             Surface Area                                                                          Milled                                                                            Annealed                                                                           Ash                           (cm.sup.3 /g)                                                                           (A)  (m.sup.2 /g)                                                                      (cm.sup.3 /g)                                                                      (A)    (cm.sup.3 /g)                                                                         MI  Density                                                                            (ppm)                         __________________________________________________________________________    Production of Ethylene-Hexene-1 Copolymers                                    Example                                                                       II   2.66 347  307 2.48 330    305     4.0 0.956                                                                              250                           Control                                                                       B    --   --   --  1.44 225    226     1.0 0.956                                                                              150                           Production of Ethylene-Butene-1 Copolymers                                    Example                                                                       III  2.66 347  307 2.12 235    242     2.50                                                                              0.9430                                                                             72                            Control                                                                       C    --   --   --  1.60 213    300     0.14                                                                              0.9425                                                                             70                            __________________________________________________________________________

It will be seen from Table I and II that use of the stereospecificcatalysts hereof in ethylene particle form polymerizations producespolymers having significantly greater melt indexes than polymersobtained utilizing prior art silica gel-supported catalysts, at the sameactivity levels.

It will be understood that various changes may be made in thepolymerization techniques and catalyst compositions exemplifiedhereinabove without departing from the scope of the present invention.Accordingly, the preceding specification is intended as illustrativeonly, and not in a limiting sense.

What is claimed is:
 1. A particle form polymerization process for thepolymerization of an olefin, which comprises contacting a 1-olefinmonomer, said 1-olefin having a maximum of 8 carbon atoms and nobranching nearer the double bond than the 4-position, with a supportedcatalyst comprising a metal-containing catalytic material deposited on asilica xerogel support, said silica xerogel havinga. a nitrogen porevolume greater than 1.96 cc/g and up to 2.90 cc/g, said pore volumebeing equal to the volume of the pores in said gel having pore diametersof up to 600 A and being determined as that volume of nitrogen adsorbedby and condensed in the pores of said gel per gram of the dry gel at thenormal boiling point of liquid nitrogen and at a relative pressure P/Poequal to 0.967 wherein P is the pressure of the nitrogen vapor over thegel and Po is the vapor pressure of liquid nitrogen; b. the majorportion of said nitrogen pore volume being provided by pores having porediameters within the range of from 300-600 A; and c. a surface areawithin the range of from 200-500 m² /g; and maintaining the reactionmixture containing the monomer and catalyst in slurry form and undertemperature and pressure conditions effective for polymerization.
 2. Theparticle form polymerization process of claim 1, wherein said 1-olefinmonomer is ethylene.
 3. The particle form polymerization process ofclaim 1, wherein said metal-containing catalytic material is a metaloxide selected from the group consisting of an oxide of chromium,cobalt, nickel, vanadium, molybdenum, and tungsten.
 4. The particle formpolymerization process of claim 1, wherein said metal containingcatalytic material is chromium oxide in an amount of from 0.5 to 5percent by weight, based on the total weight of the supported catalyst.5. The particle form polymerization process of claim 1, wherein thenitrogen pore volume of the silica xerogel catalyst support is withinthe range of from 2.35 cc/g to 2.90 cc/g.
 6. A particle formpolymerization process for the production of an ethylene polymer, whichcomprises contacting ethylene with a supported catalyst comprisingchromium oxide deposited on a silica xerogel support, the silica xerogelhavinga. a nitrogen pore volume greater than 1.96 cc/g and up to 2.90cc/g, said pore volume being equal to the volume of the pores in saidgel having pore diameters of up to 600 A and being determined as thatvolume of nitrogen adsorbed by and condensed in the pores of said gelper gram of the dry gel at the normal boiling point of liquid nitrogenand at a relative pressure P/Po equal to 0.967 wherein P is the pressureof the nitrogen vapor over the gel and Po is the vapor pressure ofliquid nitrogen; b. the pore volume of the xerogel being provided bypores having a narrow pore diameter distribution primarily within therange of from 300-600 A; and c. a surface area within the range of from200-500 m² /g; wherein the chromium oxide is deposited on said supportin an amount of from 0.5 to 5 percent by weight, based on the totalweight of the supported catalyst; maintaining the reaction mixturecontaining the monomer and catalyst in slurry form and under temperatureand pressure conditions effective for polymerization; and recovering thepolymer product.